Phase-locked loop (PLL) devices are control systems that generate signals having a fixed relationship to the phase of a reference signal. Typically, a phase-locked loop device generates a desired signal in response to both the frequency and the phase of the reference signal as well as a control signal. Often this includes raising or lowering the frequency of a frequency generator, such as a digitally controlled oscillator (DCO), a voltage controlled oscillator (VCO), or the like, until a true or modified form (a fraction, for example) of the oscillator output signal is matched with the reference signal in both frequency and phase. Phase-locked loops are widely used in radio, telecommunications, computers, and other electronic applications.
Controlled oscillators may experience jitter, or variations in the timing of the rising and/or falling edges of the periodic signal. Accumulated jitter can cause in-band phase noise, and like negative effects. Some oscillator types (such as ring oscillators, for example) may experience a greater amount of jitter than others at particular frequencies or with some applications, making them less desirable for high-accuracy applications. Additionally, timing issues with correction circuits (such as regulating loops, for instance) used with oscillators to mitigate the effects of temperature and/or supply variations can increase the occurrence of reference spurs and jitter.